Description
Radar is an acronym for "radio detection and ranging." A radar
system usually operates in the ultra-high-frequency (UHF) or
microwave part of the radio-frequency (RF) spectrum, and is used
to detect the position and/or movement of objects. Radar can
track storm systems, because precipitation reflects
electromagnetic fields at certain frequencies. Radar can also
render precise maps. Radar systems are widely used in
air-traffic control, aircraft navigation, and marine navigation.

High-power radar, using large dish antennas, has been used to
measure distances to the moon, other planets, asteroids, and
artificial satellites. From unmanned space probes, radar has
been used to map Venus, whose surface is obscured at visible
wavelengths by a thick layer of clouds. Radar has been employed
by NASA (the U.S. National Aeronautics and Space Administration)
to make highly detailed topographical maps of the earth's
surface as well.

Most radar systems determine position in two dimensions: azimuth
(compass bearing) and radius (distance). The display is in polar
coordinates. A rotating antenna transmits RF pulses at defined
intervals. The delay between a transmitted pulse and the echo,
or return pulse, determines the radial position of the plotted
point(s) for each azimuth direction on the display. The greater
the echo delay from a particular object in space, the farther
from the display center its point appears. The maximum range of
a UHF or microwave radar system depends on the height of the
antenna above average terrain, the topography of the surface in
the region, the atmospheric conditions in the region, and in
some cases the level of radio background noise.

Radar is known to the general public for its use by law
enforcement in determining the speeds of motor vehicles. This
type of radar does not display the exact position of an object,
but determines its radial speed vector from the Doppler
effect. A radar detector, which consists of a simple
UHF/microwave broadband receiver, can be used in a car or truck
to warn drivers of the presence of police radar. Radar detectors
are illegal in some states.

The Weather Service uses so-called Doppler radar to determine
not only the positions and extent of storm systems, but wind
patterns and velocities aloft. Doppler radar employs a
combination of position-sensing and speed-sensing radar, making
it possible to ascertain the locations and intensity of severe
thunderstorms, hurricanes, and tornadoes.

Radar has been used on the high-frequency (HF) radio bands,
between approximately 5 MHz and 20 MHz, in an attempt to obtain
early warning in the event of a nuclear assault via ballistic
missiles. The ionosphere refracts HF waves, allowing much
greater system range than is possible with radar at UHF or
microwave frequencies. During the 1970s and early 1980s, the
signals from these systems became infamous because of the
interference they caused. Radio amateurs coined the term
woodpecker to describe the sound of HF over-the-horizon radar
pulses in communications receivers.